Conventionally, a surface acoustic wave device to which filters having different center frequencies are provided on one piezoelectric substrate, particularly, a surface acoustic wave duplexer (hereinafter, referred to as a SAW duplexer) is known as the one having a structure shown in FIG. 14. FIG. 14 is a top view showing the inside of a package of the conventional SAW duplexer. In FIG. 14, SAW duplexer 300 includes surface acoustic wave element 304 (hereinafter, referred to as SAW element), package 305 which houses the element, wire lead 307 which connects between them, and a cover not shown. In SAW element 304, transmission filer 302 and reception filter 303 are formed on a surface of piezoelectric substrate 301 made of a lithium tantalate (LiTaO3) single crystal substrate and the like. After housing SAW element 304 in package 305, connection terminal patterns 3021, 3031 of transmission filter 302 and reception filter 303 are connected to terminal portions 306 of package 305 by wire leads 307 to obtain electrical conduction, then, the not-shown cover is sealed to package 305 to fabricate the device.
In such SAW duplexer 300, lower costs, smaller and thinner device are further required, and it is required that SAW element 304 which integrally forms transmission filter 302 and reception filter 303 is further miniaturized. However, if a shape of SAW element 304 is miniaturized, electromagnetic interference occurs between transmission filter 302 and reception filter 303, therefore, there is a problem that the isolation property deteriorates.
With respect to the problem, a SAW duplexer having good isolation property even when miniaturized is disclosed in Unexamined Japanese Patent Publication No. 2000-335143. The SAW duplexer includes a package in which a transmission terminal, a reception terminal, an antenna terminal and a ground terminal are formed, a phase shifter formed inside the package and one end of which is connected to the antenna terminal, a transmission filter mounted inside the package and an input side of which is connected to the transmission terminal and an output side of which is connected to the input terminal of the phase shifter, and a reception filter mounted inside the package and an input side of which is connected to an output terminal of the phase shifter and an output side of which is connected to the reception terminal, in which one terminal of the phase shifter is provided at the same end portion as the antenna terminal formed in the package and the other terminal of the phase shifter is provided at the opposite end portion of the one terminal of the phase shifter. According to this, the distance between the input terminal and the output terminal of the phase shifter can be long, therefore, electromagnetic interference of transmission signals and reception signals therebetween can be prevented.
In the disclosed example, the SAW element connects the transmission filter and the reception filter formed on the same piezoelectric substrate by the phase shifter provided in the package, thereby preventing electromagnetic interference. However, the prevention of electromagnetic interference by means of the SAW element itself or a connection structure of wire leads and the like for connecting from the SAW element to the package is not shown at all.
An object of the invention is to provide a small surface acoustic wave device having good filter properties, as well as which is low in cost, in which the isolation property is improved while preventing electromagnetic interference by providing a shield electrode between filter structures having two different center frequencies.